The French defender's transfer was announced in May and, after his contract in Belgium expired, he has now made the switch to Serie A.
The 23-year-old has signed a five-year contract and was presented as coach Rafael Benitez's first signing of the close-season on Tuesday.
After two seasons playing in the Belgian Pro League, the defender is looking forward to testing himself in a new environment when the season begins in August.
"I'm thrilled to be playing with Napoli," he told the club's official website. "[They are] a club that has a big, important project.
"I'm ready for this adventure. I am looking forward to starting work with my new team-mates."
Napoli have also confirmed that goalkeeper Roberto Colombo and full-back Giandomenico Mesto have agreed fresh terms at Stadio San Paulo.